Apparatus for removal of pollution from gas stream

ABSTRACT

An improved thermal oxidizer ( 10 ) comprising a combustion chamber ( 62 ), a refractory coated mixing device ( 14 ) within a plenum ( 12 ), a burner ( 26 ) mounted outside the oxidizer ( 10 ) for ready access, and temperature sensing and control equipment ( 86 ). The oxidizer ( 10 ) uses the mixing device ( 14 ) to induce a static pressure drop between the burner ( 26 ) and the oxidizer inlet ( 16 ), and a flow passage conveys preheated gas from the plenum ( 12 ) to the burner ( 26 ). A bend in the combustion chamber ( 92 ) provides for recirculation of combustion gases for more efficient burning. The burner ( 26 ) can be a commercially available unit that can accommodate inlet temperatures of up to 1200° F., allowing efficient operation of the thermal oxidizer ( 10 ).

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the removal of pollution from gas streams, and more particularly, to a direct flame incinerator for thermally oxidizing pollutants within an oxygen-bearing, process gas stream, before the process gas stream is vented to the atmosphere.

[0002] The removal of pollutants and undesirable substances from the gas streams of various manufacturing processes is well known in the prior art. Such undesirable substances include impurities and undesirable byproducts. Release or emission of these substances often must be controlled to conform to requirements of the Clean Air Act.

[0003] One conventional approach for removing or controlling these substances is by thermal oxidization. Thermal oxidation occurs when the process gas, containing sufficient oxygen, is heated to sufficient temperatures for a sufficient length of time. Thermal oxidization converts these substances to the harmless gases carbon dioxide and water vapor.

[0004] It is well known that increasing the level of turbulence in the combustion chamber enhances the thermal oxidation process. Turbulence can be increased by using sharp bends in the combustion chamber, or by using aerodynamic jets induced by mixing devices.

[0005] Typical thermal oxidization temperatures may range to 1800° F., and significant amounts of heat are often required. Most of the heat from the thermal oxidization process is usually recovered and used to preheat the process gas entering the thermal oxidizer. This heat is used to preheat the process gas to temperatures typically in the range of 1000 to 1200° F. Some of the heat for the thermal oxidation process comes from oxidization of the substances controlled. The remainder of the heat required for the thermal oxidization process comes from a burner fueled by either gaseous or liquid fuels.

[0006] In one type of thermal oxidizer, a fan supplies ambient air to the burner for combustion. Commercially available burners are often used with a fan supplying a fixed flow of ambient temperature air for combustion in the burner, and the thermal oxidizer temperature is controlled by varying the fuel flow. This design results in changes in burner stoichiometry as the fuel flow changes. Fortunately, these burners have a stability range wide enough to accommodate changes in burner stoichiometry and changes in thermal oxidizer process variables. Prior art places these burners outside the thermal oxidizer, allowing ready access for maintenance and service. However, these thermal oxidizers are inefficient, requiring one to supply fuel to heat the ambient temperature burner air to the thermal oxidizer operating temperature.

[0007] More efficient thermal oxidizers use oxygen in the preheated process gas stream to supply oxygen for burner combustion. Such oxidizers can save approximately 35% in fuel costs compared to oxidizers using ambient air for burner combustion. These prior art thermal oxidizers, however, require that a specialized burner be located inside the preheated process gas duct. These thermal oxidizers expose the burner, and other equipment such as dampers, to the preheated process gas stream where the high temperatures often cause mechanical failure. Commercially available and inexpensive, refractory-lined, carbon steel burners cannot be used in the preheated gas stream. Design, development and manufacture of specialized burners for these thermal oxidizers is expensive and time consuming. Also, locating the burner inside the process air duct hinders maintenance and servicing of the burners and other equipment.

[0008] An example of the prior art using process gas oxygen for burner combustion is U.S. Pat. No. 4,444,735 issued to Birmingham, et al. on Apr. 24, 1984. However, the thermal oxidizer in Birmingham, et al. has a complex control system with dampers operating in the preheated process gas stream. Metal dampers are prone to failure due to excessive inlet temperatures that may reach 1200° F. The Birmingham, et al. thermal oxidizer also uses a perforated metal mixing device as part of the burner. The mixing device improves combustion by enhancing turbulence in the combustion chamber, and mixing of the preheated process gas with the burner flame. Such mixing devices are commonly used in thermal oxidizers. However, the mixing device used by Birmingham, et al. is prone to overheating and mechanical failure due to high inlet temperatures and exposure to thermal radiation from the hot combustion gases. For this reason, the mixing device material temperature is used as a burner control system variable to limit the temperatures in that oxidizer. If the mixing device were protected with a suitable refractory coating, this limitation could be avoided.

[0009] A similar thermal oxidizer to that in Birmingham, et al. is disclosed in U.S. Pat. No. 4,444,724 issued to Goetschius on Apr. 24, 1984. The Goetschius oxidizer also has a metal mixing device incorporated with the burner, and the mixing device is cooled with process air. This complicated burner is costly to produce. The burner is also difficult to replace or service, because it is located inside the preheated process air duct.

[0010] The thermal oxidizer disclosed in U.S. Pat. No. 5,762,880 issued to Rühl, et al. on Jun. 9, 1998, also uses process gas oxygen for burner combustion. This thermal oxidizer uses instrumentation to measure combustion gas and fuel flow, valves or dampers to control gas flows and fuel flow, and an electronic control system to precisely control burner stoichiometry. The burner also may move relative to the combustion chamber to control airflow to the burner. Such a complicated measurement and control system is expensive and unreliable. For many thermal oxidizers, precise burner stoichiometry control is unnecessary.

SUMMARY OF THE INVENTION

[0011] The primary objects and advantages of the present invention are:

[0012] to use preheated, process gas to supply oxygen for combustion in a burner, resulting in substantial fuel savings;

[0013] to use a mixing device to increase turbulence in the combustion chamber for improved thermal oxidizer performance;

[0014] to protect the mixing device with a refractory coating to improve the reliability of the mixing device;

[0015] to use the mixing device as a baffle in the thermal oxidizer to provide sufficient pressure to force the process gas through a burner, to eliminate a separate combustion air source;

[0016] to use a conventional, commercially available burner, located outside the thermal oxidizer, allowing ready access for service and maintenance; and

[0017] to locate the mixing device inside the thermal oxidizer for efficient flow of process gas through the mixing device and through the burner, and to incorporate a bend in the combustion chamber to promote turbulence and efficiently direct combustion gases to a heat exchange means.

[0018] The present invention fulfills the above and other objects by providing a thermal oxidizer for removing combustible substances from an oxygen bearing gas stream, having a combustion chamber containing at least one bend whereby the turbulence within the combustion chamber is improved and the flow direction of the to combustion gases is reversed so that the gases are efficiently removed from the combustion chamber and directed to a heat exchanger. A mixing device located at the entrance to the combustion chamber and disposed axially to the combustion chamber contains a plurality of holes allowing the flow of gas into the combustion chamber. A burner assembly located external to the thermal oxidizer discharges into the mixing device. A flow passage from a plenum to the inlet of the burner allows oxygen bearing gas to be supplied from the plenum. The combustion chamber has gas temperature sensing means and a control unit varies the fuel flow to the burner to obtain and maintain desired operating temperatures within the combustion chamber. The mixing device within the thermal oxidizer may be a metal duct, having a plurality holes in the sides to allow the flow of gas through the wall of the duct, with a refractory coating.

[0019] The above and other objects, features and advantages of the present invention should become even more readily apparent to those skilled in the art upon a reading of the following detailed description in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In the following detailed description, reference will be made to the attached drawings in which:

[0021]FIG. 1 shows a sectional view of the thermal oxidizer and heat exchange means; and

[0022]FIG. 2 shows a sectional view of the thermal oxidizer taken through lines 2-2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] For purposes of describing the preferred embodiment, the terminology used in reference to the numbered components in the drawings is as follows: 8. Process Gas 44. Burner Flame 10. Thermal Oxidizer 46. Jets of Polluted Process Air 12. Plenum 50. Plenum Refractory Coating 14. Mixing Device 52. Combustion Chamber 16. Thermal Oxidizer Inlet Refractory 20. Heat Exchange Means 54. Mixing Device Refractory 22. Mixing Device Hole 58. Mixing Device Metal Shell 24. Gas Flow Passage from 59. Mixing Device End Plate Plenum to Burner Inlet 60. Metal Lining for Hole in 26. Burner Refractory 30. Burner Air Inlet 62. Combustion Chamber 32. Burner Fuel Inlet 64. Combustion Gas 34. Heat Exchange Means Cold 76. Control Unit Side Inlet 86. Combustion Chamber 36. Heat Exchange Means Cold Temperature Sensing Means Side Exhaust 92. Bend in Combustion 38. Heat Exchange Means Hot Chamber Side Exhaust 94. Heat Exchange Means Hot 42. Burner Fuel Control Valve Side Inlet

[0024] The thermal oxidizer system of the present invention is illustrated in FIGS. 1 and 2. Referring to FIG. 1, process gas 8 enters heat exchange means 20. The heat exchange means 20 has a cold, or process gas side, and a hot, or combustion gas side. The cold gas side has an inlet 34 and an exhaust 36, and the hot gas side has an inlet 94 and an exhaust 38. As the process gas traverses the heat exchange means 20, it is passed in indirect heat exchange relationship with combustion gas 64 leaving the combustion chamber 62. The process gas 8 is thereby preheated, whereby combustion efficiency is increased and the heat required to operate the thermal oxidizer is decreased. The preheated process gas leaves the heat exchange means 20 at the cold side exhaust 36 and enters the thermal oxidizer 10 at the oxidizer inlet 16.

[0025] The thermal oxidizer has a vertical mixing device 14 mounted in a plenum 12. The preheated process gas enters the plenum 12 and flows around the mixing device 14. Some of the heated process gas leaves the plenum 12 through holes 22 in the mixing device 14. The remainder of the process gas leaves the plenum 12 through a gas flow passage 24. The gas flow passage 24 may be part of the plenum 12, or it may be a separate pipe or duct. The passage 24 directs preheated process gas to a burner 26.

[0026] Ready access to the burner 26 and related equipment is provided for maintenance and servicing. The burner 26 has an air inlet 30 and fuel inlet 32. Fuel is supplied to the burner 26 through fuel control valve 42, which is connected to a fuel supply that is not shown. Fuel burned in the burner incinerates the process gas in combustion chamber 62, producing combustion gas 64. Combustion chamber 62 has a bend 92 to direct combustion gases from the mixing device 14 to hot-side inlet 94 of heat exchange means 20. The combustion gas then leaves the combustion chamber 62 and enters the heat exchange means 20. After passing through the heat exchange means 20, the combustion gas leaves the heat exchange means through exhaust 38 and is vented to the atmosphere through a stack that is not shown.

[0027] The plenum 12 has a refractory coating 50. Combustion chamber 62 has a refractory coating 52.

[0028] The mixing device 14, shown in FIG. 2, comprises a metal shell 58, metal end plate 59, and a refractory coating 54. The mixing device 14 incorporates holes 22 that form jets 46 of process gas. The jets promote turbulence and mixing of the process gas and burner flame, and thereby enhance combustion.

[0029] Combustion chamber temperature is measured by temperature sensing means 86. Control unit 76 uses this temperature measurement to adjust the fuel flow through burner fuel control valve 42.

[0030] Thus, the thermal oxidizer 10 has a burner 26, a mixing device 14, and a combustion chamber 62. The inlet 16 of the thermal oxidizer 10 is connected to the cold-side exhaust 36 of a heat exchanger 20. The discharge of the combustion chamber 62 is connected to hot-side inlet 94 of the heat exchanger 20. The cold-side inlet 34 of the heat exchanger 20 is connected to a process gas supply, which is not shown.

[0031] The mixing device 14 is mounted in a plenum 12. The heat exchanger 20 is set at an angle of approximately 90° to the axis or centerline of the mixing device. Aligning the heat exchanger 20 at a right angle to the mixing device 14 allows the burner 26 to be located outside the oxidizer 10, plenum 12 and inlet duct. The burner 26 is axially disposed to the mixing device 14, and the mixing device 14 is axially disposed to combustion chamber 62.

[0032] The mixing device 14 comprises a metal shell 58, a metal end plate 59, and a refractory coating 54.

[0033] Plenum 12 is a chamber or cavity of sufficient volume to house the mixing device 14, and provides uniform static pressure, and adequate gas flow, around the mixing device 14. A flow passage 24 directs preheated process gas from the plenum to air inlet 30 of burner 26. The flow passage may be a pipe or a duct connected to the plenum, or it may be part of the plenum as shown in FIG. 1. The passage may be located anywhere on the plenum. However, a desirable location for the flow passage is the side of the plenum 12 opposite the plenum inlet. This location allows the process gas to flow completely around the mixing device 14 and cool its outer metal shell 58.

[0034] During operation the mixing device 14 induces a decrease in static pressure between the plenum 12 and the combustion chamber 62, and forms jets 46 of process gas that mix with the flame 44 from the burner 26. The jet-mixing process increases turbulence in both the mixing device 14 and the combustion chamber 62, enhancing incineration of the pollutants and undesirable substances in the process gas. The decrease in static pressure also induces process gas to flow through flow passage 24 and through burner 26, and into the mixing device 14 and combustion chamber 62. The mixing device 14 thus provides the means to induce polluted, process air through the burner 26, eliminating the separate combustion air fan and motor used in the prior art.

[0035] Holes 22 in metal shell 58 and refractory 54 may be formed using any of a number of various techniques which use the metal shell to accurately and reliably size the holes. The holes must be sized accurately and reliably, because the hole sizes influence the static pressure differences across the holes and across burner 26. Consistent static pressure differences are required for consistent burner performance and consistent mixing of gas jets 46 and burner flame 44. The holes 22 shown in FIGS. 1 and 2 are made using sections of metal pipe or tube 60 to line the hole 22 in the refractory 54. The metal lining in the hole aids in forming the refractory during manufacture, and enhances durability of the refractory 54. Without the metal lining, relatively cool air jets flowing through the holes 22 may cause erosion of the refractory 54, or destructive thermal stresses and cracks in the hot refractory. Both erosion and thermal stresses eventually cause loss of refractory which may result in damage to metal shell 58 or plate 59. Loss of refractory near holes 22 may change the geometry of the holes 22 and thereby change the coefficient of discharge of the holes 22. A change in the coefficient of discharge of the holes 22 may result in a change of gas flow, or a change in static pressure difference across the holes 22. Thus, accurate and reliable sizing of holes 22 results in consistent and reliable operation of the thermal oxidizer 10.

[0036] Jets 46 are located to allow for complete combustion of the fuel supplied to the burner 26. The jets 46 of process gas are located around the periphery of the mixing device 14, so jets 46 impinge on each other near the center. Making the jets 46 impinge near the center of the mixing device 14, or combustion chamber 62, eliminates excessive penetration of the jets 46. Excessive penetration may lead to jet impingement on a wall. The impingement of a relatively cool gas jet on a hot refractory wall may damage the refractory by erosion and by inducing thermal stresses, with undesirable results as discussed above.

[0037] It is well known in the prior art that impinging jets 46 result in higher levels of turbulence than a single jet. High levels of turbulence enhance combustion efficiency and thermal destruction of undesirable substances in the process gas. Generally, the more jets 46 and the higher the jet velocity, the higher the level of turbulence. However, the more jets 46, the smaller the diameter of each jet or the lower the jet velocity, for a given airflow. The smaller the jet or the lower the jet velocity, the shorter the distance the jet will penetrate, other factors being equal.

[0038] The number and size of jets 46 are chosen to ensure sufficient penetration of the process gas jets into the mixing device 14 and combustion chamber 62, for mixing with the burner flame. Jet penetration varies with the difference in static pressure across the holes 22, and an unnecessarily high difference in static pressure leads to unnecessarily high power consumption. Jet penetration distance also varies with the size of the holes 22, and velocities and densities of the process gas and the combustion gas. Jet penetration may be calculated using any one of a number of correlations used in the prior art. Some of the jet holes may be sized differently to penetrate different distances than others, for an even distribution of process gas in the combustion chamber 62.

[0039] The numbers and sizes of holes 22 in mixing device 14 are chosen to also provide adequate static pressure difference for burner 26. Calculations indicate that static pressure differences that yield adequate jet penetration distances are also adequate for proper burner operation.

[0040] Refractory 54 may be a castable material similar to cement, or a fibrous refractory, or any other type of refractory. The refractory thickness is sized to provide sufficient insulation to protect metal parts from burner flame 44.

[0041] The burner 26 is typically a commercially available burner that will accommodate inlet temperatures of up to 1200° F. Most of these burners are made of inexpensive carbon steel and lined with refractory for reliable, high-temperature operation. The burner may be fueled with a gaseous fuel or a liquid fuel, and some burners can operate on either fuel. The burner 26 is located outside the plenum 12 for ready access to the burner and other equipment requiring periodic service and maintenance. Locating the burner outside the process gas duct also allows the burner to be made of inexpensive, low-temperature carbon steel.

[0042] The burner fuel control valve 42 regulates the fuel supply to the burner 26. Control unit 76 determines the position of the fuel supply valve 42, and thereby the fuel flow to the burner 26. Combustion chamber 62 temperature is measured with temperature sensing means 86. The measured combustion chamber 62 temperature is compared to a desired temperature set in the control unit, and the control unit sets the fuel control valve position to maintain the desired temperature.

[0043] The preferred embodiment uses a bend or turn 92 in combustion chamber 62, to route combustion gases to the heat exchange means hot-side inlet 94. The turn increases turbulence and mixing in the combustion chamber 62 near the turn and downstream of the turn, which enhances combustion in that section of the combustion chamber 62. This bend 92 also results in a combustion chamber 62 folded to a compact size, and easy entry to the heat exchange means 20 inlet. An alternative embodiment, familiar to those skilled in the art, is to eliminate bends in the combustion chamber 62 and redirect the combustion chamber 62 gases toward the heat exchange means with a duct containing a sufficient number of bends.

[0044] In operation, the process gas stream to be incinerated is conveyed through the heat exchange means 20 and thermal oxidizer 10 under the influence of a forced draft fan, which is not shown, disposed upstream of the cold side inlet of the heat exchange means 20. Alternatively, the process gas stream may be conveyed through the heat exchange means 20 and thermal oxidizer 10 under the influence of an induced draft fan, which is not shown, located at the exhaust 38 of the hot-side of the heat exchange means 20.

[0045] From the description above, a number of advantages of the present invention become evident:

[0046] (a) process gas oxygen is used for combustion in the burner;

[0047] (b) a burner is located outside the thermal oxidizer for ready maintenance and service;

[0048] (c) the mixing device uses a refractory lining, resulting in extended oxidizer life when compared to components without refractory coatings;

[0049] (d) there are no moving parts in the heated process gas stream, resulting in a reliable control system;

[0050] (e) turbulence and mixing are promoted throughout the combustion chamber, resulting in enhanced combustion; and

[0051] (f) the combustion products are efficiently routed through the combustion chamber and the heat exchange means, with no external ducting required.

[0052] Accordingly, the reader will see that the present thermal oxidizer uses process gas to supply oxygen for combustion in a burner, eliminating a separate combustion air source and resulting in substantial fuel savings. In addition, an effective and reliable mixing device provides turbulence in the combustion chamber 62 for excellent oxidizer performance. Also, a conventional, commercially available burner is used and located outside the thermal oxidizer for ready access and maintenance.

[0053] Although only a few embodiments of the present invention have been described in detail hereinabove, all improvements and modifications to this invention within the scope or equivalents of the claims are included as part of this invention. 

I claim:
 1. A thermal oxidizer for removing combustible substances from an oxygen-bearing gas stream comprising: a combustion chamber containing at least one bend, whereby turbulence within the combustion chamber is improved, and whereby the flow direction of combustion gases is reversed and said combustion gases are efficiently removed from said combustion chamber and directed to a heat exchange means; a mixing device located within a plenum so the axis of said mixing device is approximately perpendicular to the direction of gas flow into said plenum, said mixing device located at the entrance to said combustion chamber and said mixing device disposed axially with said combustion chamber, said mixing device containing a plurality of holes allowing flow of gas into said combustion chamber, whereby a static pressure difference between said plenum and said combustion chamber is created; a burner assembly discharging into said mixing device, said burner assembly located external to the thermal oxidizer and disposed axially to said mixing device and said combustion chamber, whereby ready access to the burner assembly and its components is allowed; a flow passage from said plenum to the air inlet of said burner, whereby said air inlet is supplied with oxygen-bearing gas from said plenum; a combustion chamber gas temperature sensing means; and a control means to vary the fuel flow to said burner, whereby a desired operating temperature in the combustion chamber is achieved and maintained.
 2. The thermal oxidizer of claim 1, wherein said mixing device comprises: a metal duct, protected by a refractory lining, with a metal plate on one end that is also protected by a refractory lining; a plurality of holes located in the sides of said duct, allowing the flow of gas through the wall of the duct, with at least one pair of said holes sized to form gas jets that meet near the centerline of said mixing device; and means to protect the refractory lining from erosion, thermal stresses and cracking, whereby the operational life of the refractory is extended and consistent operation of the thermal oxidizer is maintained.
 3. A thermal oxidizer comprising: a combustion chamber with means for changing the direction of flow of combustion chamber gases, whereby said gases are directed toward the inlet of said oxidizer; a mixing device located within the inlet of said oxidizer, disposed axially to the inlet of said combustion chamber, with a plurality of holes located in the walls of said mixing device, whereby a difference in static pressure between said oxidizer inlet and said combustion chamber is created to induce the flow of gas to a burner and induce gas jets which enhance turbulence and mixing within said thermal oxidizer; burner located external to the thermal oxidizer and disposed axially to said mixing device and the inlet of said combustion chamber; a gas flow passage from the oxidizer inlet to the burner air inlet; gas temperature sensing means; and a control means to control the flow of fuel to said burner, whereby a desired operating temperature in the combustion chamber is achieved and maintained. 